Absolute vibrational and electronic cross sections for low-energy electron (2-12 eV) scattering from condensed pyrimidine

Low-energy vibrational and electronic electron-energy-loss (EEL) spectra of pyrimidine condensed on a thin film of solid argon held at 18 K are reported for the incident-energy range of 2-12 eV. Sensitivity to symmetry and spin forbidden transitions as well as correlations to the triplet states of benzene make it possible to ascribe the main features, below 7 eV in the electronic part of the EEL spectrum, to triplet transitions. The lowest EEL feature with an energy onset at 3.5 eV is attributed to a transition to the (3)B(1)(n-->pi(*)) valence electronic state and the next triplet n-->pi(*) transition to a (3)A(2) state located around 4.5 eV. The remaining EEL features at 4.3, 5.2, 5.8, and 6.5 eV are all assigned to pi-->pi(*) transitions to states of symmetry (3)B(2), (3)A(1), (3)B(2), and (3)B(2)+(3)A(1), respectively. The most intense maximum at 7.6 eV is found to correspond to both (1)B(2) and (1)A(1) transitions, as in the vacuum ultraviolet spectra. Absolute inelastic cross sections per scatterer are derived from a single collision treatment described herein. Their values are found to lie within the 10(-17) cm(2) range for both the electronic and the vibrational excitations. Features in the energy dependence of the cross sections are discussed, whenever possible, by comparison with data and mechanisms found in the gas phase. A maximum over the 4-5 eV range is attributed to a B (2)B(1) shape resonance and another one observed in the 6-7 eV range is ascribed to either or both sigma(*) shape resonances of (2)A(1) and (2)B(2) symmetries.     

A new pathway for the rapid decay of electronically excited adenine

Combined density functional and multireference configuration interaction methods have been used to calculate the electronic spectrum of 9H-adenine, the most stable tautomer of 6-aminopurine. In addition, constrained minimum energy paths on excited potential energy hypersurfaces have been determined along several relaxation coordinates. The minimum of the first (1)[n-->pi*] state has been located at an energy of 4.54 eV for a nuclear arrangement in which the amino group is pyramidal whereas the ring system remains planar. Close by, another minimum on the S(1) potential energy hypersurface has been detected in which the C(2) center is deflected out of the molecular plane and the electronic character of S(1) corresponds to a nearly equal mixture of (1)[pi-->pi*] and (1)[n-->pi*] configurations. The adiabatic excitation energy of this minimum amounts to 4.47 eV. Vertical and adiabatic excitation energies of the lowest n-->pi* and pi-->pi* transitions as well as transition moments and their directions are in very good agreement with experimental data and lend confidence to the present quantum chemical treatment. On the S(1) potential energy hypersurface, an energetically favorable path from the singlet n-->pi* minimum toward a conical intersection with the electronic ground state has been identified. Close to the conical intersection, the six-membered ring of adenine is strongly puckered and the electronic structure of the S(1) state corresponds to a pi-->pi* excitation. The energetic accessibility of this relaxation path at about 0.1 eV above the singlet n-->pi* minimum is presumably responsible for the ultrafast decay of 9H-adenine after photoexcitation and explains why sharp vibronic peaks can only be observed in a rather narrow wavelength range above the origin. The detected mechanism should be equally applicable to adenosine and 9-methyladenine because it involves primarily geometry changes in the six-membered ring whereas the nuclear arrangement of the five-membered ring (including the N(9) center) is largely preserved.     

Theoretical study of the ground and excited states of 7-methyl guanine and 9-methyl guanine: comparison with experiment

The keto-enol tautomerization of 7-methyl-guanine and 9-methyl-guanine in the excited state was investigated using the time-dependent DFT (TDDFT) method. For both species, the potential energy surfaces of the ground state and two lowest singlet excited states (due to pi-->pi* and n-->pi* transitions) have been investigated and their features discussed in terms of consequences on the excited state dynamics. The findings suggest that, for both species, the state due to the n-->pi* transition, suspected to be an intermediate in the excited state deactivation, exhibits two minima with the second minimum characterized by an elongated N1-H distance. This structure, intermediate between enol and keto tautomers, might play a role in the excited state relaxation. The existence of this second well, however, is observed in both 7- and 9-methyl-guanine, which suggests that it cannot account alone for the different photophysical behavior of these species.     

Electronic excitations of fluoroethylenes

Several lowest-lying singlet electronic states of vinyl fluoride, trans-, cis-, and 1,1-difluoroethylene, trifluoroethylene, and tetrafluoroethylene were investigated by using symmetry-adapted cluster configuration interaction theory. Basis sets up to Dunning's aug-cc-pVTZ augmented with appropriate Rydberg functions were utilized for the calculations. Calculated excitation energies show a good agreement with the available experimental values. Even in the troublesome pi-->pi(*) transitions, the excitation energies obtained in the present study agree well with the experimental values except in one or two fluoroethylenes. Strong mixing between different states was noticed in a few fluoroethylenes; especially the mixing is very strong between pi-pi(*) and pi-3ppi states in trifluoroethylene. No pure pi-sigma(*) excited state was found in almost all the fluoroethylenes. Several assignments and reassignments of features in the experimental spectra were suggested. The present study does not support the existing argument that the interaction between the pi-pi(*) and sigma-sigma(*) states is the reason behind the blueshift of around 1.25 eV in the pi-pi(*) excitation energy of tetrafluoroethylene. Possible reasons, including structural changes, for this shift are discussed in detail. Several low-lying triplet excited states were also studied.     

Effects of chain length and Au spin-orbit coupling on 3(pi pi*) emission from bridging Cn2- units: theoretical characterization of spin-forbidden radiative transitions in metal-capped one-dimensional carbon chains [H3PAu(C[triple bond]C)nAuPH3

Density functional theory and CASSCF calculations have been used to optimize the geometries of binuclear gold(I) complexes [H(3)PAu(C[triple bond]C)(n)AuPH(3)] (n=1-6) in their ground states and selected lowest energy (3)(pi pi*) excited states. Vertical excitation energies obtained by time-dependent density functional calculations for the spin-forbidden singlet-triplet transitions have exponential-decay size dependence. The predicted singlet-triplet splitting limit of [H(3)PAu(C[triple bond]C)(proportional/variant)AuPH(3)] is about 8317 cm(-1). Calculated singlet-triplet transition energies are in reasonable agreement with available experimental observations. The effect of the heavy atom Au spin-orbit coupling on the (3)(pi pi*) emission of these metal-capped one-dimensional carbon allotropes has been investigated by MRCI calculations. The contribution of the spin- and dipole-allowed singlet excited state to the spin-orbit-coupling wave function of the (3)(pi pi*) excited state makes the low-lying acetylenic triplet excited states become sufficiently allowed so as to appear in both electronic absorption and emission.     

Photoinduced omega-bond dissociation of m-halomethylbenzophenones studied by laser photolysis techniques and DFT calculations. Substituted position effects

Photochemical profiles of omega-cleavage of carbon-X (X = Br and Cl) bonds in m-bromo- and m-chloromethylbenzophenones (m-BMBP and m-CMBP) were investigated by laser photolysis techniques and DFT calculations. m-BMBP and m-CMBP were found to undergo omega-bond cleavage to yield the m-benzoylbenzyl radical (m-BBR) at 295 K, and the quantum yields were determined. No CIDEP signal was detected upon 308 nm laser photolysis of both the compounds. From these observations, it was inferred that the omega-bond of these m-halomethylbenzophenones (m-HMBP) cleaves in the lowest excited singlet state (S(1)(n,pi(*))) upon direct excitation. Upon triplet sensitization of acetone (Ac), the m-BBR formation was observed in transient absorption for an Ac-m-BMBP system, and an efficiency of the C-Br bond cleavage in the lowest triplet state (T(1)(n,pi(*))) of m-BMBP was determined. In contrast, formation of triplet m-CMBP was seen for an Ac-m-CMBP system. Absence of C-Cl bond cleavage in the triplet state of m-CMBP indicated the reactive state of m-CMBP for omega-cleavage is only the S(1)(n,pi(*)) state. Based on the efficiencies and DFT calculations for excited state energies, photoinduced omega-bond dissociation of m- and p-HMBPs was characterized.     

Theoretical calculation about the valence and Rydberg excited states of hydrogen cyanide

The singlet and triplet excited states of hydrogen cyanide have been computed by using the complete active space self-consistent field and completed active space second order perturbation methods with the atomic natural orbital (ANO-L) basis set. Through calculations of vertical excitation energies, we have probed the transitions from ground state to valence excited states, and further extensions to the Rydberg states are achieved by adding 1s1p1d Rydberg orbitals into the ANO-L basis set. Four singlet and nine triplet excited states have been optimized. The computed adiabatic energies and the vertical transition energies agree well with the available experimental data and the inconsistencies with the available theoretical reports are discussed in detail.     

Computational analyses of singlet-singlet and singlet-triplet transitions in mononuclear gold-capped carbon-rich conjugated complexes

Density functional theory and CASSCF calculations have been used to determine equilibrium geometries and vibrational frequencies of metal-capped one-dimensional pi-conjugated complexes (H3P)Au(C[triple chemical bond]C)(n)(Ph) (n = 1-6), (H3P)Au(C[triple chemical bond]CC6H4)(C[triple chemical bond]CPh), and H3P--Au(C[triple chemical bond]CC6H4)C[triple chemical bond]CAu--PH3 in their ground states and selected low-lying pi(pi)* excited states. Vertical excitation energies for spin-allowed singlet-singlet and spin-forbidden singlet-triplet transitions determined by the time-dependent density functional theory show good agreement with available experimental observations. Calculations indicate that the lowest energy 3(pi(pi)*) excited state is unlikely populated by the direct electronic excitation, while the low-lying singlet and triplet states, slightly higher in energy than the lowest triplet state, are easily accessible by the excitation light used in experiments. A series of radiationless transitions among related excited states yield the lowest 3(pi(pi)*) state, which has enough long lifetimes to exhibit its photochemical reactivities.     

Photophysics of aromatic molecules with low-lying pi sigma* states: fluorinated benzenes

Unlike fluorinated benzenes with four or less fluorine atoms, pentafluorobenzene (PFB) and hexafluorobenzene (HFB) exhibit very small fluorescence yields and short fluorescence lifetimes. These emission anomalies suggest that the nature of the first excited singlet (S(1)) state may be different for the two classes of fluorobenzenes. Consistent with this conjecture, the time-dependent density-functional theory calculations yield S(1) state of pi pi(*) character for fluorinated benzenes with four or less F atoms, and S(1) state of pi sigma(*) character for PFB and HFB. The pi sigma(*) character of the S(1) state of PFB and HFB has been confirmed by laser-induced fluorescence, which reveal the presence of a new electronic transition to the red of the (1)pi pi(*) (L(b))<--S(0) transition, which can be identified with the predicted low-energy (1)pi sigma(*)<--S(0) absorption. The low fluorescence yields and the short fluorescence lifetimes of PFB and HFB are consistent with the small radiative decay rate of the (1)pi sigma(*) state and efficient S(1) (pi sigma(*))-->S(0) internal conversion between two electronic states of very different geometries.     

Ab initio calculations of triplet excited states and potential-energy surfaces of vinyl chloride: insights into spectroscopy and photodissociation dynamics

The equilibrium geometries and harmonic vibrational frequencies of three low-lying triplet excited states of vinyl chloride have been calculated using the state-averaged complete active space self-consistent field (CASSCF) method with the 6-311++G(d,p) basis set and an active space of four electrons distributed in 13 orbitals. Both adiabatic and vertical excitation energies have been obtained using the state-averaged CASSCF and the multireference configuration-interaction methods. The potential-energy surfaces of six low-lying singlet states have also been calculated. While the 3(pi, pi*) state has a nonplanar equilibrium structure, the 3(pi, 3s) and 3(pi, sigma*) states are planar. The calculated vertical excitation energy of the 3(pi, pi*) state is in agreement with the experiment. The singlet excited states are found to be multiconfigurational, in particular, the first excited state is of (pi, 3s) character at the planar equilibrium structure, of (pi, sigma*) as the C-Cl bond elongates, and of (pi, pi*) for highly twisted geometries. Avoided crossings are observed between the potential-energy surfaces of the first three singlet excited states. The absorption spectra of vinyl chloride at 5.5-6.5 eV can be unambiguously assigned to the transitions from the ground state to the first singlet excited state. The dissociation of Cl atoms following 193-nm excitation is concluded to take place via two pathways: one is through (pi, sigma*) at planar or nearly planar structures leading to fast Cl atoms and the other through (pi, pi*) at twisted geometries from which internal conversion to the ground state and subsequent dissociation produces slow Cl atoms.     

Experimental and theoretical study of the photodissociation reaction of thiophenol at 243 nm: intramolecular orbital alignment of the phenylthiyl radical

The photoinduced hydrogen (or deuterium) detachment reaction of thiophenol (C(6)H(5)SH) or thiophenol-d(1) (C(6)H(5)SD) pumped at 243 nm has been investigated using the H (D) ion velocity map imaging technique. Photodissociation products, corresponding to the two distinct and anisotropic rings observed in the H (or D) ion images, are identified as the two lowest electronic states of phenylthiyl radical (C(6)H(5)S). Ab initio calculations show that the singly occupied molecular orbital of the phenylthiyl radical is localized on the sulfur atom and it is oriented either perpendicular or parallel to the molecular plane for the ground (B(1)) and the first excited state (B(2)) species, respectively. The experimental energy separation between these two states is 2600+/-200 cm(-1) in excellent agreement with the authors' theoretical prediction of 2674 cm(-1) at the CASPT2 level. The experimental anisotropy parameter (beta) of -1.0+/-0.05 at the large translational energy of D from the C(6)H(5)SD dissociation indicates that the transition dipole moment associated with this optical transition at 243 nm is perpendicular to the dissociating S-D bond, which in turn suggests an ultrafast D+C(6)H(5)S(B(1)) dissociation channel on a repulsive potential energy surface. The reduced anisotropy parameter of -0.76+/-0.04 observed at the smaller translational energy of D suggests that the D+C(6)H(5)S(B(2)) channel may proceed on adiabatic reaction paths resulting from the coupling of the initially excited state to other low-lying electronic states encountered along the reaction coordinate. Detailed high level ab initio calculations adopting multireference wave functions reveal that the C(6)H(5)S(B(1)) channel may be directly accessed via a (1)(n(pi),sigma(*)) photoexcitation at 243 nm while the key feature of the photodissociation dynamics of the C(6)H(5)S(B(2)) channel is the involvement of the (3)(n(pi),pi(*))-->(3)(n(sigma),sigma(*)) profile as well as the spin-orbit induced avoided crossing between the ground and the (3)(n(pi),sigma(*)) state. The S-D bond dissociation energy of thiophenol-d(1) is accurately estimated to be D(0)=79.6+/-0.3 kcalmol. The S-H bond dissociation energy is also estimated to give D(0)=76.8+/-0.3 kcalmol, which is smaller than previously reported ones by at least 2 kcalmol. The C-H bond of the benzene moiety is found to give rise to the H fragment. Ring opening reactions induced by the pi-pi(*)n(pi)-pi(*) transitions followed by internal conversion may be responsible for the isotropic broad translational energy distribution of fragments.     

Theoretical investigation of electron transfer transition in tetracyanoethylene-contained organic complexes

In this work, the authors use complete active space self-consistent field method to investigate the photoinduced charge-separated states and the electron transfer transition in complexes ethylene-tetracyanoethylene and tetramethylethylene-tetracyanoethylene. Geometries of isolated tetracyanoethylene, ethylene, and tetramethylethylene have been optimized. The ground state and the low-lying excited states of ethylene and tetracyanoethylene have been optimized. The state energies in the gas phase have been obtained and compared with the experimentally observed values. The torsion barrier of tetracyanoethylene has been investigated through the state energy calculation at different conformations. Attention has been particularly paid to the charge-separated states and the electron transfer transition of complexes. The stacked conformations of the donor-acceptor complexes have been chosen for the optimization of the ground and low-lying excited states. Equilibrium solvation has been considered by means of conductor-like screening model both in water and in dichloromethane. It has been found that the donor and tetracyanoethylene remain neutral in complexes in ground state (1)A(1) and in lowest triplet state (3)B(1), but charge separation appears in excited singlet state (1)B(1). Through the correction of nonequilibrium solvation energy based on the spherical cavity approximation, pi-->pi* electron transfer transition energies have been obtained. Compared with the experimental measurements in dichloromethane, the theoretical results in the same solvent are found higher by about 0.5 eV.     

Excitation Energies of Canonical Nucleobases Computed by Multiconfigurational Perturbation Theories

In this computational work, we assessed the performance of ab initio multireference (MR) methods for the calculation of vertical excitation energies of five nucleobases: adenine, guanine, cytosine, thymine and uracil. In total, we have studied 38 singlet and 30 triplet excited states. Where possible we used the multireference configuration interaction (MRCI) method as a reference for various flavors of multireference perturbation theory to second order. In particular, we have benchmarked CASPT2, NEVPT2 and XMCQDPT2. For CASPT2, we have analyzed the single‐state, multistate (MS) and extended MS variants. In addition, we have assessed the effect of the ionization potential electron affinity (IPEA) shift. For NEVPT2, we have used the partially and the strongly contracted variants. Further, we have tested the commonly used RI‐CC2, RI‐ADC2 and EOM‐CCSD methods. Generally, we observe the following trends for singlet excited states: NEVPT2 is the closest MR method to MRCISD+Q, closely followed by CASPT2 with the default IPEA shift. The same trend is observed for triplet states, although NEVPT2 and CASPT2‐IPEA are getting closer. Interestingly, the n, π* singlet excited states were described more accurately than π, π* excited states, while for triplet states the trend is inverted except for NEVPT2. This work is an important benchmark for future photochemical investigations.     

Excited state properties of 7-hydroxy-4-methylcoumarin in the gas phase and in solution. A theoretical study

TDDFT/B3LYP and RI-CC2 calculations with different basis sets have been performed for vertical and adiabatic excitations and emission properties of the lowest singlet states for the neutral (enol and keto), protonated and deprotonated forms of 7-hydroxy-4-methylcoumarin (7H4MC) in the gas phase and in solution. The effect of 7H4MC-solvent (water) interactions on the lowest excited and fluorescence states were computed using the Polarizable Continuum Method (PCM), 7H4MC-water clusters and a combination of both approaches. The calculations revealed that in aqueous solution the pi pi* energy is the lowest one for excitation and fluorescence transitions of all forms of 7H4MC studied. The calculated excitation and fluorescence energies in aqueous solution are in good agreement with experiment. It was found that, depending on the polarity of the medium, the solvent shifts vary, leading to a change in the character of the lowest excitation and fluorescence transition. The dipole-moment and electron-density changes of the excited states relative to the ground state correlate with the solvation effect on the singlet excited states and on transition energies, respectively. The calculations show that, in contrast to the ground state, the keto form has a lower energy in the pi pi* state as compared to enol, demonstrating from this point of view the energetic possibility of proton transfer from the enol to the keto form in the excited state.     

The electronic structure and spectra of Ru(II) and Ru(III) complexes with imidazole and its derivatives

The calculations of the electronic structure and spectra of [Ru(NH3)5L]2+ (L = imidazole, histidine) and [Ru(NH3)5L]3+ (L = imidazole, N-imidazolate anion, 4-methylimidazole, 4-methyl-1N-imidazolate anion and 1N-bound histidine) complexes are performed in the framework of the CI method in the INDO/CNDO approximation. The MO diagram is obtained. The assignment of all transitions with energies of 4-5 eV is made and the nature of corresponding excited states is discussed. For the Ru(II) complexes, the lower energy observable transition is assigned to d-->pi* type, whereas the higher energy one is assigned to pi-->pi* type. In the spectra of the Ru(III) complexes with charged ligands both transitions are of pi-->d character, while in the case of uncharged ligands, the higher energy transition mostly incorporates pi-->pi* excitations.     

Phosphorescence characteristics of acetophenone, benzophenone, p-aminobenzophenone and Michler's ketone in various environments

The phosphorescence characteristics (excitation and emission spectra and lifetimes) of acetophenone (AP), benzophenone (BP), p-aminobenzophenone (PABP) and Michler's ketone (MK) adsorbed on Whatman No. 1 filter paper were measured at various temperatures, and compared with the phosphorescence characteristics in different solvent glasses at 77 K. Both AP and BP phosphoresce on filter paper only at low temperature (208 K). The phosphorescence lifetimes of AP and BP are < 1 msec, indicating a (3)(n,pi(*)) lower triplet level for paper substrates. With PABP, the low lying triplet state in polar solvents is (3)(CT) and in non-polar solvents is (3)(n, pi(*)); PABP on filter paper results in spectral characteristics similar to those of PABP in polar solvents at 77 K. The lifetime of PABP is longer than that of BP, indicating a (3)(CT) low-lying triplet state. MK, like PABP, has strongly environment-dependent photophysical properties. MK, when adsorbed on filter paper, has an intense long-lived luminescence at room temperature, resulting in a limit of detection of 3 ng ml or 3 pg, and a linear dynamic range of over 3 orders of magnitude. MK appears to be strongly hydrogen-bonded to the filter paper. In studies in ethanol and other solvents, MK adsorbed on filter paper shows a dramatic change in its phosphorescence spectrum when the temperature is lowered from 298 K to 208 K; the phosphorescence peak moves to longer wavelengths and the intensity decreases. The temperature effect could arise from the presence of several conformers of MK or be due to different environmental sites or E-type delayed fluorescence. The low-lying triplet state of MK on filter paper is most likely a (3)(CT) state. Lowering the temperature appears to increase the phosphorescence intensity for ketones which phosphoresce in the (3)(n,pi(*)) triplet state, but affects it only slightly for analytes which phosphoresce in the (3)(pi,pi(*)) triplet state. Room-temperature phosphorescence seems to arise for aromatic ketones and aldehydes with low-lying (3)(pi, pi(*)) or (3)(CT) triplet states.     

Spectroscopy and transport of the triplet exciton in a terthiophene end-capped poly(phenylene ethynylene)

Triplet states of poly(phenylene ethynylene), (3)PPE(*), not easily formed by direct photoexcitation, were produced by pulse radiolysis in toluene, along with triplet states of T(3)PPE having terthiophene end-caps. Intense triplet-triplet absorption maxima, epsilon(680)((3)PPE(*)) = 9.5 x 10(4) M(-1) cm(-1) and epsilon(780)((3)T(3)PPE(*)) = 2.8 x 10(4) M(-1) cm(-1) enable identification of these two species, which have triplet energies of 2.12 and 1.77 eV determined in bimolecular energy transfer equilibria. Bleaching of ground-state absorption measures (3)PPE(*) to be the delocalized over a 1.8-nm length. Triplet states formed in the PPE chains were transported to and trapped by the end caps in a time <5 ns.     

On the performance of quantum chemical methods to predict solvatochromic effects: the case of acrolein in aqueous solution

The performance of the Hartree-Fock method and the three density functionals B3LYP, PBE0, and CAM-B3LYP is compared to results based on the coupled cluster singles and doubles model in predictions of the solvatochromic effects on the vertical n-->pi* and pi-->pi* electronic excitation energies of acrolein. All electronic structure methods employed the same solvent model, which is based on the combined quantum mechanics/molecular mechanics approach together with a dynamical averaging scheme. In addition to the predicted solvatochromic effects, we have also performed spectroscopic UV measurements of acrolein in vapor phase and aqueous solution. The gas-to-aqueous solution shift of the n-->pi* excitation energy is well reproduced by using all density functional methods considered. However, the B3LYP and PBE0 functionals completely fail to describe the pi-->pi* electronic transition in solution, whereas the recent CAM-B3LYP functional performs well also in this case. The pi-->pi* excitation energy of acrolein in water solution is found to be very dependent on intermolecular induction and nonelectrostatic interactions. The computed excitation energies of acrolein in vacuum and solution compare well to experimental data.     

Triplet state properties and triplet-state-oxygen interactions of some linear and angular furocoumarins

Linear and angular furocoumarins with conjugated external carbonyl substituents show higher triplet and singlet oxygen yields than the corresponding unsubstituted molecules. The efficiency of the oxygen quenching process to yield singlet oxygen is also higher for these substituted molecules. These changes are interpreted in terms of the "proximity effect" associated with two nearly degenerate n pi* and pi pi* excited states, and variations in the excess energy following furocoumarin triplet quenching by ground state triplet oxygen to yield singlet oxygen.     

Change in spin state and enhancement of redox reactivity of photoexcited states of aromatic carbonyl compounds by complexation with metal ion salts acting as Lewis acids. Lewis acid-catalyzed photoaddition of benzyltrimethylsilane and tetramethyltin via photoinduced electron transfer

The lowest excited state of aromatic carbonyl compounds (naphthaldehydes, acetonaphthones, and 10-methylacridone) is changed from the n,pi triplet to the pi,pi singlet which becomes lower in energy than the n,pi triplet by the complexation with metal ions such as Mg(ClO(4))(2) and Sc(OTf)(3) (OTf = triflate), which act as Lewis acids. Remarkable positive shifts of the one-electron reduction potentials of the singlet excited states of the Lewis acid-carbonyl complexes (e.g., 1.3 V for the 1-naphthaldehyde-Sc(OTf)(3) complex) as compared to those of the triplet excited states of uncomplexed carbonyl compounds result in a significant increase in the redox reactivity of the Lewis acid complexes vs uncomplexed carbonyl compounds in the photoinduced electron-transfer reactions. Such enhancement of the redox reactivity of the Lewis acid complexes leads to the efficient C-C bond formation between benzyltrimethylsilane and aromatic carbonyl compounds via the Lewis-acid-promoted photoinduced electron transfer. The quantum yield determinations, the fluorescence quenching, and direct detection of the reaction intermediates by means of laser flash photolysis experiments indicate that the Lewis acid-catalyzed photoaddition reactions proceed via photoinduced electron transfer from benzyltrimethylsilane to the singlet excited states of Lewis acid-carbonyl complexes.